Multi-phase buck converters have been widely used for various applications, such as power supply circuits. For example, as shown in FIG. 1, a four-phase buck converter 10 has four output stages each corresponding to one phase thereof and including a pair of MOSFETs connected in series, i.e., MOSFETs 121h and 121l, 122h and 122l, 123h and 123l, and 124h and 124l, manipulated by respective drivers 141, 142, 143 and 144 in respective phases, which are controlled by a control logic 16. To obtain stable and balanced output in each phase, an error amplifier 18 is employed to compare a reference voltage VREF with the output voltage VOUT of the converter 10 to thereby produce a voltage error signal supplied to four pulse width modulation (PWM) comparators 201, 202, 203 and 204, each of which has its another input connected with the current error signal in respective phase. For generation of these current error signals, four current sense signal generators 221, 222, 223 and 224 are connected with the voltage PH1, PH2, PH3, and PH4, respectively, and ground in respective phase to produce four current sense signals for each phase to be further summed by a summing circuit 24 and averaged by a averaging circuit 26 to obtain an average current signal, and four subtracting circuits 281, 282, 283 and 284 subtract the average current signal from the current sense signals, respectively. These current error signals from the subtracting circuits 281, 282, 283 and 284 are further added with a ramp signal from a ramp generator 32 by four summing circuits 301, 302, 303 and 304, respectively, and the resulted signals are sent to the four PWM comparators 201-204 to be compared with the voltage error signal from the error amplifier 18 to thereby produce four feedback signals for the control logic 16 so as to control the drivers 141-144 to switch the MOSFETs 121-124 of the output stages to produce the converter output voltage VOUT and converter output current Iout on and from the output of the converter 10 through four output inductances L1, L2, L3 and L4 in each phase and output capacitance 34.
The converter 10 converts the energy supplied from the power supply with the input voltage VIN to the output VOUT by uniformly sharing the current to each phase and therefore generates four phase currents IQ1, IQ2, IQ3 and IQ4 and four phase voltages PH1-PH4 in each phase, respectively. FIG. 2 shows the waveforms of the phase voltages PH1-PH4 and phase currents IQ1-IQ4 in each phase and input current Ivin from the input node VIN. In FIG. 2A, waveform 361 is the phase voltage PH1 of phase 1, waveform 362 is the phase voltage PH2 of phase 2, waveform 363 is the phase voltage PH3 of phase 3, and waveform 364 is the phase voltage PH4 of phase 4. In FIG. 2B, waveforms 381, 382, 383 and 384 are the phase currents IQ1-IQ4 in each phase, respectively, and their sum is represented by waveform 40 for the input current Ivin shown in FIG. 2C. Among them the duty in each phase depends on the ratio of the output voltage to input voltage, i.e., VOUT/VIN. The output stages in each phase produce respective line currents IL1, IL2, IL3 and IL4 through the output inductances L1-L4 and their sum Iout, i.e., the converter output current, as shown in FIG. 3. In FIG. 3A, waveform 421 is the line current IL1 of phase 1, waveform 422 is the line current IL2 of phase 2, waveform 423 is the line current IL3 of phase 3, and waveform 424 is the line current IL4 of phase 4. Waveform 44 shown in FIG. 3B represents the converter output current Iout. Although this scheme obtains stable converter output voltage VOUT and balanced phase currents IQ1-IQ4 between phases, the converter output current Iout has severe ripples and the behavior of the ripples relate to the duty in each phase. Unfortunately, the rippled output is disadvantageous to the load of the converter. Therefore, it is desired a ripple-free multi-phase buck converter. Further, the more is the difference between the input voltage VIN and the converter output voltage VOUT, the poorer efficiency has the converter. Also, higher voltage and faster switching speed are necessary for the MOSFETs of the output stages when the difference between the input voltage VIN and the converter output voltage VOUT becomes more. Accordingly, it is further desired a multi-phase buck converter with improved conversion efficiency and lower voltage and lower cost output stages and drivers.